If Apples battery efficiency is so good they should release a phone with a 5000mah battery /as well as/ said efficiency and market their battery life that beats all their competitors by multiple days, I'm sure a lot of people would buy that in a heartbeat.
The downsides of larger batteries are increased weight, volume, and cost. Being more efficient gets the same battery life for less of those three, allowing them to either be reduced or the budget to go to other components.
As for Xiaomi being comically large, I found the mid-range Samsung and Motorola models to have larger bezels and to generally be larger for similar specs when I bought my Xiaomi Redmi Note 7 a couple years ago. The reality is that the majority of phones from Motorola, Xiaomi, LG and Nokia are designed and manufactured by 3 Chinese ODMs (Wingtech, Huaquin and Longcheer), and even 20% of Samsung's phones come from these 3 ODMs. See: https://amosbbatto.wordpress.com/2021/12/10/comparing-l5-and...